Many of my friends have taken pets on KE as they are teachers living here that move back to the US. They claim the general 5kg rule is flexible as long as it "fits under the seat."
I would imagine that flexibility also depends on the airline staff when you check in/who staffs your counter. Everyone that's said "oh they're flexible" has also departed from ICN.
But, if you get the Delta agent at ATL, for instance, they might not be flexible. When I flew to Korea in August 2011, it was proper Korean Air staff. Last summer, Delta staff. This summer, who knows.
Good luck!